The present invention relates to a length or angle measuring instrument for measuring the relative position of first and second objects movable relative to one another, of the type comprising a measuring scale or graduation carrier adapted to be mounted to the first object, and a scanning unit adapted to be mounted to the second object.
It is a generally known practice to provide machine tools with so-called limit position switches, which are actuated by cams fastened to the slide piece of the machine tool. As suggested by the name limit position switch, when the slide piece reaches a limit position the switch is controlled to generate a limit position signal. In general, this signal is used as an emergency off signal for the machine tool. When the cam and the follower of the switch come into contact at a predetermined limit position, the switch is activated. See for example the discussion in Simon, "Die numerische Steuerung von Werkzeugmaschinen" page 7, 1971 Edition. Because the moving machine parts have a relatively large mass and a over-run on the machine tool commonly results, it is also common practice to provide preliminary switching points at which the speed of the machine slide piece is reduced. These preliminary switching point switches are likewise arranged to be actuated by cams.
In prior art devices in which limit switches and cams are mounted to the machine tool itself, precisely processed assembling surfaces must be provided on the machine tool. Such precisely processed assembling surfaces require space on the machine tool and increase the cost of the machine tool.